Cemented carbide insert

ABSTRACT

The present invention relates to a cutting tool insert and its methods of manufacture for machining of steel comprising a cemented carbide body and a coatings. The cemented carbide body includes WC, 5-12 wt-% Co and 3-11 wt-% of cubic carbides of metals Ta and Ti. The amount of Nb is below 0.1 wt-% and the ratio Ta/Ti is 1.0-4.0. The Co-binder phase is highly alloyed with W with a CW-ratio of 0.75-0.95 and, finally, the cemented carbide body has a binder phase enriched and essentially gamma phase free surface zone of a thickness of 5-50 μm.

CROSS-REFERENCES TO RELATED APPLICATIONS

The present application is a reissue of U.S. Pat. No. 6,344,264 B1,which claims the benefit of priority to Swedish Application No.9901243 - 7 filed Apr. 8, 1999.

BACKGROUND OF THE INVENTION

The present invention relates to a coated cemented carbide cutting toolinsert particularly useful for turning operations in steels or stainlesssteels, and is especially suited for operations with high demandsregarding toughness properties of the insert.

High performance cutting tools must nowadays possess high wearresistance, high toughness properties and good resistance to plasticdeformation. Improved toughness behaviour of a cutting insert can beobtained by increasing the WC grain size and/or by raising the overallbinder phase content, but such changes will simultaneously result insignificant loss of the plastic deformation resistance.

Methods to improve the toughness behaviour by introducing thickessentially gamma phase-free and binder phase-enriched surface zone witha thickness of about 20-40 μm on the inserts by a so-called “gradientsintering” techniques have been known for some time e.g. U.S. Pat. No.4,277,283, U.S. Pat. No. 4,497,874, U.S. Pat. No. 4,548,786, U.S. Pat.No. 4,610,931, U.S. Pat. No. 5,484,468, U.S. Pat. No. 5,549,980, U.S.Pat. No. 5,649,279, U.S. Pat. No. 5,729,823. The characteristics ofthese patents are that the surface zone has a different composition thanthe bulk composition, and is depleted of gamma phase and binder phaseenriched.

SUMMARY OF THE INVENTION

It has now surprisingly been found that by using a gamma phaseconsisting essentially of only TaC and TiC in addition to WC, by keepingthe ratio between the elements Ta and Ti within specific limits, andhaving a highly W-alloyed binder phase, the toughness properties of thegradient sintered cutting inserts can be significantly improved withoutany loss of plastic deformation resistance.

A first aspect of the present invention provides a cutting tool insertfor machining steel comprising a cemented carbide body comprising WC,5-12 wt. % Co, 3-11 wt. % of cubic carbides of the metals Ta and Ti, andless than 0.1 wt. % of Nb where the ratio of Ta/Ti is 1.0-4.0, and theCo-binder phase is highly alloyed with W, having a CW-ratio of0.75-0.95, the body also comprising a binder phase enriched andessentially gamma phase free surface zone with a thickness of 5-50 μm;and a coating.

A second aspect of the present invention provides a method of making acoated cemented carbide body having a gamma phase-free and binder richsurface zone comprising the steps of:

-   -   (i) forming a powder mixture comprising WC, 5-12 wt. % Co, 3-11        wt. % cubic carbides of Ta and Ti, where the ratio of Ta/Ti is        1.0-4.0;    -   (ii) adding N in an amount of 0.6-2.0% of the weight of Ta and        Ti;    -   (iii) milling and spray drying the mixture to form a powder        material with the desired properties;    -   (iv) compacting and sintering the powder material at a        temperature of 1300-1500° C., in a controlled atmosphere of        about 50 mbar followed by cooling, whereby a body having a        binder phase enriched and essentially gamma phase free surface        zone of 5-50 μm in thickness is obtained;    -   (v) applying a pre-coating treatment to the body; and    -   (vi) applying a hard, wear resistant coating.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a plot showing the level of Co enrichment near the surface ofan insert formed according to the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

According to the present invention there is now provided a coatedcemented carbide insert with a 5-50 μthick, preferably 10-30 μm thick,essentially gamma phase free and binder phase-enriched surface zone withan average binder phase content (by volume) preferably in the range1.2-2.0 times the bulk binder phase content.

The gamma phase consists essentially of TaC and TiC and of any WC thatdissolves into the gamma phase during sintering. The ratio Ta/Ti isbetween 1.0 and 4.0, preferably 2.0-3.0.

The binder phase is highly W-alloyed. The content of W in the binderphase can be expressed as aCW-ratio=M_(s)/(wt. Col*0.0161) whereM_(s) is the measured saturation magnetization of the cemented carbidebody in kA/m hAm² /kg and wt-% Co is the weight percentage of Co in thecemented carbide. The CW-ratio takes a value less than or equal to 1.The lower the CW-ratio, the higher the W-content in the binder phase. Ithas now-been found according to the invention that an improved cuttingperformance is achieved if the CW-ratio is in the range 0.75-0.95,preferably 0.80-0.85.

The present invention is applicable to cemented carbides with acomposition of 5-12, preferably 9-11, weight percent of Co binder phase,and 3-11, preferably 7-10, weight percent TaC+TiC, and the balance beingWC. The Nb content should not exceed 0.1 weight percent. The weightratio Ta/Ti should be 1.0-4.0, preferably 2.0-3.0. The WC preferably hasan average grain size of 1.0 to 4.0 μm, more preferably 1.5 to 3.0 μm.The cemented carbide body may contain less than 1 volume % of η-phase(M₆C).

Inserts according to the invention are further provided with a coatingpreferably comprising 3-12 μm columnar TiCN-layer followed by a 1-8 μmthick Al₂O₃-layer deposited, for example, according to any of thepatents U.S. Pat. No. 5,766,782, U.S. Pat. No. 5,654,035, U.S. Pat. No.5,974,564, U.S. Pat. No. 5,702,808, preferably a κ-Al₂O₃-layer andpreferably with an outermost thin layer of TiN which preferably isremoved in the edge line by brushing or by blasting.

According to the invention, by applying coatings with differentthickness on the cemented carbide body the property of the coated insertcan be optimised to suit specific cutting conditions.

In one embodiment, a cemented carbide insert produced according to theinvention is provided with a coating of: 6 μm TiCN, 5 μm Al₂O₃ and 1 μmTiN. This coated insert is particularly suited for cutting operation insteel.

In another embodiment, a cemented carbide insert produced according tothe invention is provided with a coating of: 4 μm TiN, 2 μm Al₂O₃ and 1μm TiN. This coating is particularly suited for cutting operations instainless steels.

The invention also relates to a method of making cutting insertscomprising a cemented carbide substrate of a binder phase of Co, WC, agamma phase of Ta and Ti, a binder phase enriched surface zoneessentially free of gamma phase, and a coating. A powder mixturecontaining 5-12, preferably 9-11, weight percent of binder phaseconsisting of Co, and 3-11, preferably 7-10, weight percent TaC+TiC, andthe balance WC with an average grain size of 1.0-4.0 μm, more preferably1.5-3.0 μm, is prepared. The Nb content should not exceed 0.1 weightpercent. The weight ratio Ta/Ti should be 1.0-4.0, preferably 2.0-3.0.Well-controlled amounts of nitrogen have to be added either the powderas carbonitrides and/or added during the sintering process via thesintering gas atmosphere. The amount of nitrogen added will determinethe rate of dissolution of the cubic phases during the sintering processand hence determine the overall distribution of the elements in thecemented carbide after solidification. The optimum amount of nitrogen tobe added depends on the composition of the cemented carbide and, inparticular, on the amount of cubic phases and varies between 0.6 and2.0% of the weight of the elements Ti and Ta. The exact conditionsdepend to a certain extent on the design of the sintering equipmentbeing used. It is within the purview of the skilled artisan to determinewhether the requisite surface zone of the cemented carbide have beenobtained and to modify the nitrogen addition and the sintering processin accordance with the present specification in order to obtain thedesired result.

The raw materials are mixed with pressing agent and, optionally W, suchthat the desired CW-ratio is obtained. The mixture is milled and spraydried to obtain a powder material with the desired properties. Next, thepowder material is compacted and sintered. Sintering is performed at atemperature of 1300-1500° C., in a controlled atmosphere of about 50mbar followed by cooling. After conventional post sintering treatments,including edge rounding, a hard, wear resistant coating according toabove is deposited by CVD- or MT-CVD-technique.

EXAMPLE 1

A.) Cemented carbide turning inserts of the style CNMG 120408-PM andSNMG120412-PR with the composition 9.9 wt % Co, 6.0 wt % TaC, 2.5 wt %TiC, and 0.3 wt % TiN, with the balance WC having an average grain sizeof 2.0 μm were produced according to the invention. The nitrogen wasadded to the carbide powder as TiCN. Sintering was done at 1450° C. in aatmosphere of Ar at a total pressure of about 50 mbar.

Metallographic investigation showed that the inserts had a gamma phasefree zone of 15 μm. FIG. 1 shows a plot of the Co enrichment near thesurface measured by an image analysis technique. The Co is enriched to apeak level of 1.3 times the bulk content. Magnetic saturation valueswere recorded and used for calculating CW-values. An average CW-value of0.81 was obtained.

After a pre-coating treatment like edge honing, cleaning etc. theinserts were coated in a CVD-process comprising a first thin layer (lessthan 1 μm) of TiN followed by 6 μm thick layer of TICN with columnargrains by using MTCVD-techniques (process temperature 850° C. and CH₃CNas the carbon/nitrogen source). In a subsequent process step during thesame coating cycle, a 5 μm thick κ-Al₂O₃ layer was deposited accordingto U.S. Pat. No. 5,974,564. On top of the κ-Al₂O₃ layer a 1.0 μm TiNlayer was deposited. The coated inserts were brushed in order tosmoothly remove the TiN coating from the edge line.

B.) Cemented carbide turning inserts of the style CNMG 120408-PM andSNMG120412-PR with the composition 10.0 wt % Co, 2.9 wt % TaC, 3.4 wt %TiC, 0.5 wt % NbC and 0.2 wt % TiN and the balance WC with an averagegrain size of 2.1 μm were produced. The inserts were sintered in thesame process as A. Metallographic investigation showed that the producedinserts had a gamma phase free zone of 15 μm. Magnetic saturation valueswere recorded and used for calculating CW-values. An average CW-value of0.81 was obtained. The inserts were subject to the same pre-coatingtreatment as A, coated in the same coating process and also brushed inthe saute way as A.

C.) Cemented carbide turning inserts of the style CNGM 120408-PM andSNMG120412-PR with the composition 10.0 wt % Co, 3.0 wt % TaC, 6.3 wt %ZrC and balance WC with an average grain size of 2.5 μm were produced.

Metallographic investigation showed that the produced inserts had agamma phase free zone of 12 μm. Magnetic saturation values were recordedand used for calculating CW-values. An average CW-value of 0.79 wasobtained. The inserts were subject to the same pre-coating treatment asA, coated in the same coating process and also brushed in the same wayas A.

EXAMPLE 2

Inserts from A, B and C were tested with respect to toughness in alongitudinal turning operation with interrupted cuts.

Material; Carbon steel SS1312.

Cutting data: Cutting speed 130 m/min Depth of cut 1.5 mm

Feed=Starting with 0.15 mm and gradually increased by 0.10 mm/min untilbreakage of the edge

8 edges of each variant were tested

Inserts style: CNMG120408-PM

Results: Mean feed at breakage Inserts A 0.31 mm/rev Inserts B 0.22mm/rev Inserts C 0.22 mm/rev

EXAMPLE 3

Inserts from A, B and C were tested with respect to resistance toplastic deformation in longitudinal turning of alloyed steel (AISI4340).

Insert style: CNMG 120408-PM

Cutting data: Cutting speed = 100 m/min Feed = 0.7 mm/rev. Depth of cut= 2 mm Time in cut = 0.50 min

The plastic deformation was measured as the edge depression at the noseof the inserts.

Results: Edge depression, μm Insert A 49 Insert B 63 Insert C 62

EXAMPLE 4

Tests performed at an end user producing rear shaft for lorries. Theinserts from A and C were tested in a three turning operations with hightoughness demands due to interrupted cuts. The inserts were run untilbreakage of the edge. The insert style SNMG120412-PR was used. Results:

Number of machined components Operation 1 2 3 Variant A 172 219 119Variant C 20 11 50

Examples 2, 3 and 4 show that the inserts A according to the inventionsurprisingly exhibit much better toughness in combination with somewhatbetter plastic deformation resistance in comparison to conventionalinserts B and C.

The foregoing has described the principles, preferred embodiments andmodes of operation of the present invention. However, the inventionshould not be construed as being limited to the particular embodimentsdiscussed. Thus the above-described embodiments should be regarded asillustrative rather than restrictive, and it should be appreciated thatvariations may be made in those embodiments by workers skilled in theart without departing from the scope of the present invention as definedby the following claims.

1. A coated cemented carbide body comprising: a gamma phase consistingessentially of TaC, TiC and WC, wherein the ratio of Ta/Ti is 1.0-4.0,the body having a CW ratio of 0.75-0.95, the CW ratio expressed as: CWratio=M_(s)/(wt. % Co*0.0161), wherein M, M_(s) is the measuredsaturation magnetization of the body and wt. % Co is the weightpercentage of Co in the cemented carbide, the body further comprising asurface zone that is essentially gamma phase-free and is binder rich. 2.The coated body of claim 1, wherein the surface zone is approximately5-50 μm thick.
 3. The coated body of claim 1, wherein the surface zoneis approximately 10-30 μm thick.
 4. The coated body of claim 1, whereinthe surface zone has a binder phase content 1.2-2.0 times the binderphase content in the rest of the body.
 5. A coated cemented carbide bodycomprising: a gamma phase consisting essentially of TaC, TiC and WC,wherein the Ta/Ti-ratio is 2.0-3.0, the body having a CW ratio of0.75-0.95, the CW ratio expressed as: CW ratio=M_(s)/(wt. %Co^(*)0.0161), wherein M_(s) is the measured saturation magnetization ofthe body and wt. % Co is the weight percentage of Co in the cementedcarbide, the body further comprising a surface zone that is essentiallygamma phase-free and is binder rich.
 6. The coated body of claim 1,wherein the CW ratio is 0.80-0.85.
 7. The coated body of claim 1,wherein the body comprising Co content of 5-12 wt. %.
 8. The coated bodyof claim 7, wherein the Co content is 9-11 wt. %.
 9. The coated body ofclaim 1, wherein the combined content of TaC and TiC is 3-11 wt. %. 10.The coated body of claim 9, wherein the combined content of TaC and TiCis 7-10 wt. %.
 11. The coated body of claim 1, wherein the bodycomprises WC having a grain size of 1.0-4.0 μm.
 12. The coated body ofclaim 11, wherein the grain size is 1.5-3.0 μm.
 13. A coated body ofclaim 1, wherein said coating comprises a 3-12 μm columnar TiCN-layer,followed by a 1-8 μm thick Al₂O₃-layer.
 14. The coated body of claim 13,wherein the said Al₂O₃-layer is κ-Al₂O₃.
 15. The coated body of claim13, wherein the coating comprises an outermost layer of TiN.
 16. Thecoated body of claim 14, wherein the coating comprises an outermostlayer of TiN.
 17. The coated body of claim 15, having no TiN layer at anedge line of the body.
 18. The coating body of claim 1, wherein thecoating body comprises a cutting tool insert having at least one cuttingedge.
 19. The coated body of claim 5, wherein the surface zone isapproximately 5- 50 μm thick.
 20. The coated body of claim 5, whereinthe surface zone is approximately 10- 30 μm thick.
 21. The coated bodyof claim 5, wherein the surface zone has a binder phase content 1.2- 2.0times the binder phase content in the rest of the body.
 22. The coatedbody of claim 5, wherein the CW ratio is 0.80- 0.85.
 23. The coated bodyof claim 5, wherein the body comprising Co content of 5- 12 wt. %. 24.The coated body of claim 23, wherein the Co content is 9- 11 wt. %. 25.The coated body of claim 5, wherein the combined content of TaC and TiCis 3- 11 wt. %.
 26. The coated body of claim 25, wherein the combinedcontent of TaC and TiC is 7- 10 wt. %.
 27. The coated body of claim 5,wherein the body comprises WC having a grain size of 1.0- 4.0 μm. 28.The coated body of claim 27, wherein the grain size is 1.5- 3.0 μm. 29.The coated body of claim 5, wherein said coating comprises a 3- 12 μmcolumnar TiCN-layer, followed by a 1 - 8 μm thick Al ₂ O ₃-layer. 30.The coated body of claim 29, wherein the said Al₂ O ₃-layer is κ-Al ₂ O₃ .
 31. The coated body of claim 30, wherein the coating comprises anoutermost layer of TiN.
 32. The coated body of claim 29, wherein thecoating comprises an outermost layer of TiN.
 33. The coated body ofclaim 32, having no TiN layer at an edge line of the body.
 34. Thecoated body of claim 5, wherein the coated body comprises a cutting toolinsert having at least one cutting edge.